Verdant (Green) mages, a word of warning: Phoenixes although seemingly a capable unit are fraud with weaknesses so be careful in using them. When using phoenixes you will have a very low ressurection ratio and most capable attacking mages will find it easy to penetrate your defenses if you have them in first stack. Instead, use a horde of treants in conjunction with the spell Plant Growth. The higher the spell level the better the plant growth will function so always keep an eye on the black market for spells that you do not yet posses to increase your spell level. Being favored by the god 'magic' will also further increase your spell level whilst not increasing the cost or Powerrank of your Treants.
An advisable defensive assignment is Rust Armor in conjunction with either Scroll of protection from Fire or Ash of Invisibility. The Former will serve to guard your weakness against fire whilst the latter would take care of aggressors using the Spider's Web which can seriously affect your unit's initiative.
Another interesting twist verdant mages can use is to augment your forces with some ascendant units, such as archangels. Be careful, however - your first objective should be to maintain several tens of thousand of treants (as many as you can). A further augmentation to your stacking could be Dryads and Nymphs which are very powerfull units in stack 5+, where they'll not be attacked. Even non-Verdants should have 10.000 Dryads and 2.500 Nymph, so Verdant should definately too.
Note that like ascendant creatures, many verdant creatures have natural healing or regeneration abilities. While they don't work quite as effectively as ascendant creature abilities of this type, they do still work well. Enchantments on your Kingdom MUST include Plantgrowth if you are intend on using treants, additionally Sunray can be very useful as well.
As hero a high level Shaman is recommended which will further increase the strength of your Treants.